UV problem. Artifacts appearing on material
When you create new holes inside the mesh with the Boolean modifier active, Blender will wind up recalculating all of the faces affected. Normally, if you sub divide the mesh and then apply the Boolean modifier, you could get better UV textures. At this point, you can do one of the following:


Subdivide the mesh first with enough vertices to avoid this problem when you apply the Boolean modifier.
Add more vertices to certain parts of the mesh to create cleaner faces with the Boolean Modifier active.